Theory of a double-layer in an expanding electronegative plasma

Abstract

A theory is developed for formation of a weak electronegative double layer (DL) just outside of an upstream source chamber connected to a larger diameter, downstream expansion chamber. The double layer is described using four groups of charged particles: thermal positive ions, monoenergetic accelerated positive ions flowing downstream, accelerated negative ions flowing upstream, and non-Maxwellian electrons. The upstream source is modeled as a stratified plasma with an electropositive edge, and the downstream equations are solved in a spherically expanding approximation. The DL equations are solved using a non-Maxwellian electron distribution, with the solution required to be approximately self-consistent with the upstream and downstream equilibria. Particle balance upstream is used to determine the double layer potential. The theory is found to be in good agreement with experiments.